Motion detecting apparatus

ABSTRACT

Detecting apparatus for providing a signal when a rotating machine element stops and comprising a pulse generating means mechanically coupled to the machine element for producing electrical pulses at a frequency directly proportional to the speed of rotation of the element. The pulses are applied through a pulse shaper to signal generating means for producing, in response to the occurrence of each pulse, first and second signals which decrease and increase, respectively, in a regular and continuous, preferably linear, manner until the next pulse occurs whereupon the signals are repeated. The signals are applied to a comparison circuit which provides an indicating signal in response to a predetermined relationship between the first and second signals indicating that the machine element has stopped rotating, and the indicating signal is used to control the machine. In preferred form, the element is a cam fixed to the shaft of rotary switching apparatus which is coupled to the drive shaft of a machine such as a power press and which controls the machine.

United States Patent 11 1 Jones, Jr. Sept. 18, 1973 MOTION DETECTINGAPPARATUS [75] Inventor: Clarence 0. Jones, Jr., Eggertsville, [57]ABSTRACT N,Y Detecting apparatus for providing a signal when a rotatingmachine element stops and comprising a pulse [73] Asslgnee' NmgaraMachine & Tool works generating means mechanically coupled to themachine Buffalo element for producing electrical pulses at a frequency[22] Filed; on, 10, 1972 directly proportional to the speed of rotationof the element. The pulses are applied through a pulse shaper to [211App]' 296l02 signal generating means for producing, in response to theoccurrence of each pulse, first and second signals [52] US. Cl 307/120,340/271, 73/509 which decrease and increase, respectively, in a regular[51] Int. Cl. H0lh 35/06 and continuous. preferably linear, manner untilthe [58] Field of Search 307/120, 122, 119, next pulse eeeurs whereuponhe signals are repeated. 307/152, 106; 340/263, 271, 267; 73/433 Thesignals are applied to a comparison circuit which 507 509 provides anindicating signal in response to a predetermined relationship betweenthe first and second signals [5 6] Referen Cit d indicating that themachine element has stopped rotat- UNITED STATES PATENTS ing, and theindicating signal is used to control the machine. In preferred form, theelement is a cam fixed to i the shaft of rotary switching apparatuswhich is coupled to the drive shaft of a machine such as a power pressand which controls the machine.

Z '23 POWER 5 PULSE l 5AMPLE SUP LY SHAPER GENERATOR RESET 1 GENERATQRLOAD COMPARATOR CONTROL 1 MOTION DETECTING APPARATUS BACKGROUND OF THEINVENTION operate rotary switching apparatus. The switching apparatus,in conjunction with electrical circuits, can control the time in theoperating cycle and hence the position at which a machine component isstopped as well as the time when the component is again put in motion. Aparticular example of such a machine is a power press wherein the slideor ram is caused to move back and forth between a workpiece and acontrolled top stop position for each complete rotation of the pressdrive shaft. By way of illustration the presentinvention will bedescribed with particular reference to such machines, although theprinciples of the invention may be variously applied.

The machine drive shaft is coupled to rotary switching aparatus by achain, separate shaft; right angle gearing or other suitable couplingmeans, and these coupling means are susceptible to malfunction orfailure such as loosening or breaking of the chain. It therefore becomesimportant to monitor the operation of such machine control switches todetect failure or abnormal operation of the switch drive and to stop themachine in response to the same, particularly in the case of a powerpress. Prior art apparatus purporting to solve this problem monitors theoperation of the drive coupling to the switch. It would be'highlydesirable to provide apparatus for monitoring the operation of therotary switch itself which is the real item of interest, espe-' ciallyin the control of a machine such as a power press. Also, it is importantthat such monitoring apparatus be capable of operating under conditionsof variable machine drive speeds without giving any false signals.Furthermore, such monitoring apparatus should be effective and reliablein operation and reliable in operation and relatively simple andeconomical in construction.

SUMMARY OF THE INVENTION The present invention provides apparatus fordetectconstantly, and this comparison provides a signal when a pulsedoes not appear because of the fact that the niachine element hasstopped or effectively stopped rotating. This signal is used for controlpurposes such as stopping the machine.

While a single specific embodiment of the principles of the presentinvention is illustrated in the accompanying drawing and described indetail in the following specification, it is to be understood that suchembodiment is by way of example only and that various mechanicalmodifications may be made without departing froom the spirit of theinvention, the scope of which is limited only as defined in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram,partly diagrammatic, of apparatus according to the present invention fordetecting and signalling that a rotating machine element has stopped;

FIG. 2 is a graph illustrating electrical signal waveforms present atvarious locations in the apparatus of FIG. 1;

FIG. 3 is a schematic circuit diagram of one form of electrical circuit,which can be included in the apparatus of FIG. 1;

FIG. 4 is a block diagram of another electrical circuit which can beincluded in the apparatus of FIG. 1; and

FIG. 5 is a schematic diagram of the circuit of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMODIMENT FIG. 1 is a blockdiagram of apparatus according to the present invention for detectingwhen a rotating machine element stops and more particularly forsignaling a malfunction of failure in the drive to rotary switchingapparatus wherein rotary motion is derived from the drive shaft of amachine, controlled by the switch. One

form of rotary switching apparatus to which the invention isparticularly applicable is shown in US. Pat. No. 3,510,610 issued May 5,1970 and assigned to the same assignee of the present invention. In theswitching apparatus of that patent, briefly, electrical switchesattached to a housing are periodically opened and closed bycorresponding cams positioned on a shaft rotatably connected in thehousing. The shaft is coupled through suitable means to a machine driveshaft, such as the drive shaft of a power press, and the cam-operatedswitches are electrically connected in the press control circuits.

The apparatus of the present invention comprises pulse generating means,generally designated 12 in FIG. 1, mechanically coupled to a rotatingmachine element for producing electrical pulses directly proprotional tothe speed of rotation of the machine element. In the present instancethe machine element comprises a shaft 13 having a cam 14 fixed thereonand provided with a plurality of lobes or rise portions 15 located atregular or equal intervals around the circumference thereof. While theparticular number of equally-spaced cam lobes may vary depending uponthe nature of use of the present invention, in-the present example ofcontrolling a power press, six lobes 15 spaced at 60 degree angularintervals around the axis of shaft 13 is preferred. A normally-openswitch 18 is positioned in proximity to cam 14 so as to be closed by camlobes 15 coming in contact therewith, whereby switch 18 is opened-andclosed periodically at the rate of six times, i.e. six openings and sixclosings, for each rotation of shaft 13. Switch 18 is connected by lines19, 20 in an electrical circuit with a conventional electrical powersource designated 22. Therefore, in response to rotation of shaft 13pulse generating means 12 produces electrical pulses at the output 23thereof having a freduration of the pulses 24 is a function of themechanical characteristics of switch 18 and of the shape and size of camlobes 15.

The pulses 24 are applied to the input of a pulse shaping means 28wherein the pulses are converted to sharp, relatively narrower pulseshaving the same frequency as pulses 24 and illustrated by waveform 30 inFIG. 2. The purpose of this conversion is for more efficient andeffective electronic processing. Pulse shaping means 28 isrepresentative of various types of pulse shaping circuits well known tothose skilled in the art so that a detailed description thereof isbelieved to be unnecessary. Suffice it to say, pulse shaping means 28 istriggered or activated in response to the leading edge of each pulse 24for providing a pulse 30 which is relatively sharp, narrow in width, andwhich occurs at a time corresponding to that of the leading edge ofpulse 24. An illustrative form of pulse shaping means 28 will be shownin detail further on in the specification. Pulses 30 which appear at theoutput of shaping means 28 therefore have a frequency directlyproportional to the speed of rotation of shaft 13 and cam 14.

The apparatus according to the present invention further comprisessignal generating means coupled to pulse generating means 12 forproducing, in response to each pulse, first and second signals each ofwhich varies with time in a regular, continuous manner. The variationsof the first and second signals are different relative to each other butthere is a relationship between the two signals which is readilycomparable.

Each signal continues to vary until the next pulse 24 occurs whereuponthe signals are repeated and again vary in the same manner as before. Inparticular, a sample signal generator 34 is connected to the output ofpulse shaping means 28 and generates a signal only in response to thepresence of a pulse. The signal generated by means.34 has a starting orinitial value and changes with time in a regular and continuous mannerwhereupon in response to the occurrence of the next pulse 24, the signalreturns to the same starting point instantaneously and begins to. varyagain in the same manner. In preferred form the signal generated bymeans 34 starts at a predetermined level and then decreases in amplitudelinearly with time in response to the occurrence of a pulse 24 whereuponin response to the occurrence of the next pulse 24' the signal generatedby means 34 rises instantaneously to the initial or maximum amplitudeand then again begins to decrease linearly therefrom. This signal thuscomprises successive, adjacent ramp signals which combine to form asawtooth-likewaveform designated 38 in FIG. 2. The reference amplitudelevel for waveform 38 is indicated by line 39 intersecting the verticalaxis of the graph in FIG. 2.

The signal generating means further comprises a reset signal generator42 connected to sample signal generator 34. Signal generator 42functions like signal generator 34 to produce, in response to theoccurrence of a pulse 24, a signal which has a starting or initial valueand changes with time in a regular and continuous manner whereupon inresponse to the occurrence of the next pulse 24, the signal returns tothe same starting point instantaneously and begins to vary again in thesame manner. In preferred form the signal generated by means 42 startsat a predetermined level and then increases in amplitude linearly withtime in response to the occurrence of a pulse 24 whereupon in I responseto the occurrence of the next pulse 24 the signal generated by means 42falls instantaneously to the initial or minimum amplitude and then againbegins to increase linearly therefrom. The signal provided by generator42 thus differs from the signal provided by generator 34 but there is areadily comparable relationship between the two signals. This signallikewise comprises successive, adjacent ramp signals which combine toform a sawtooth-like waveform designated 46 in FIG. 2. The reference orzero level for waveform 46 is indicated by line 47 intersecting thevertical axis of the graph in FIG. 2.

Apparatus of the present invention further comprises comparison meansconnected to the signal generating means for providing an indicatingsignal in response to the occurrence of a predtermined relationshipbetween the first and second signals indicating that the machine elementhas stopped rotating. In particular, there is provided a comparatorcircuit 48 one input of which is connected by line 51 to the output ofsample signal generator 34 and the other input of which is connected byline 52 to the output of reset signal generator 42. The output ofcomparator circuit 48 is connected to a control 54. Comparator circuit50 functions, briefly, to provide an output signal only when apredetermined relationship exists between the signals appearing on lines51, 52. In the present example comparator circuit 40 will provide anoutput signal when the signals on lines 51 and 52 are coincident, thatis when the difference between the two signals is zero. When theapparatus of the present invention is used in conjunction with a machinesuch as a power press, an output signal from cpmparator 50 is utilizedto operate a relay in an emergency stop circuit of the machine control.

The apparatus of the present invention operates in the following manner.When the apparatus is used in the control of a machine such as a powerpress, cam 14 is installed on the shaft of a rotary limit switch such asthat described in the aforementioned U.S. Pat. No. 3,510,610 and shaft13 shown in FIG. 1 therefore represents the shaft in the rotary switchof that patent. For example, switch 18 in FIG. 1 can comprise onecamoperated switch of the rotary switch in which case the correspondingcam would be replaced by cam 14, or alternatively cam 14 and switch 18can be installed separately on the shaft and housing, respectively, ofthe rotary switching apparatus of that patent. The switch shaft, i.e.shaft 13 of FIG. 1, is coupled to the machine or press drive shaft bysuitable means such as a chain and sprocket coupling. Cam 14 has sixlobes or rise portions 15 spaced at 60 degree angular intervals aroundthe periphery thereof so that six pulses are produced for every rotationof shaft 13. The leading edges of the pulses 24 occur at the time whenshaft 13 rotates through the zero, sixty, 120, etc. degree angularintervals. The frequency of pulses 24 is directly proportional to thespeed of rotation of shaft 13, and so long as shaft 13 is rotated by themachine pulses 24 continue to be produced. In other words, as long asshaft 13 does not stop rotating, the next pulse in the train of pulses24 will occur at its expected point in time.

The pulses 24 are converted to the relatively narrower, sharp pulses 30which, in turn, are applied to the input of sample signal generator 34.Generator 34 produces waveform 38 which varies regularly andrepetitively in the manner illustrated in FIG. 2 so long as pulses 24continue to be produced. Similarly generator 42 produces waveform 46which varies regularly and repetitively as shown in FIG. 2 so long aspulses 24 continue to be produced. In particular, the two signalsproduced by signal generating means 34 and 42 decrease and increase,respectively, with time in a linear manner toward the same level ofmagnitude and in a manner whereby the signals do not coincide if thenext pulse 24 appears at its regular time. in the present example,pulses 24 appear at the zero, 60, 120 and 240 rotary positions of shaft13. In other words, shaft 13 continues to rotate through 240 degrees inthe present example. Then, however, shaft 13 stops rotating, for exampledue to a failure in the coupling between the machine drive shaft and theswitch shaft 13. As a result, the next pulse does not appear at itsregularly scheduled or expected time. As shown in FIG. 2, waveforms 38and 46 continue to vary in their regular, linear manner and do notreturn to their respective initial values. At a short period of timeafter the time at which the next pulse normally would have occurred atthe 360 degree rotational position of shaft 13, waveforms 38 and 46coincide whereupon comparator 50 senses this and provides an outputsignal indicating that shaft 13 has stopped rotating. This in turn wouldcause immediate shutdown of the machine or press. In particular, anoutput signal from comparator 50 causes energization of a relaycontrolling switch contacts in the emergency stop circuit of the powerpress control.

The apparatus of the present invention advantageously monitors theactual machine element of interest, for example the rotation of theshaft and cam of a rotary limit switch, thereby providing a trulyaccurate and dependable indication of a fault or malfunctionin themachine element or in the coupling of the mechanical drive thereto. Whenapplied to rotary switching apparatus controlling the operation of amachine such as a power press, the present invention uses a cam of theswitch to produce a signal which is dependent on the speed of themachine or press. The apparatus advantageously samples every pulse andit is designed so that if the next pulse does not appear at its properor regularly expected time, a malfunction or fault will be signaled. Thegeneration, in response to the occurrence of each pulse, of first andsecond signals each varying with time in a regular, continuous manneruntil the next pulse occurs whereupon the signals are repeated and againvary in the same manner as before, together with the constant comparingof the two signals to provide an indicating signalin response to theoccurrence of the predetermined relationship between the signalsindicative that rotation of the machine element has ceased, providesmonitoring apparatus which is relatively simple in construction yethighly effective and accurate in operation. The foregoing together withthe factthat a plurality of pulses, in the present instance six, aregenerated during each rotation of the machine element enables anabnormal condition to be detected and signalled extremely quickly,within a fraction of the time required for one rotation of the machinedrive shaft, which is highly desirable for reasons of safety in machinessuch as a power press. In addition, the circuit provides a signal onlywhen the machine element stops rotating and therefore is compatible withmachines drivenat variable speeds during operation. Thus the machinespeed can vary and the monitoring apparatus of the present inventionwill follow this variation, providing a signal only when the machineelement stops rotating. Therefore the apparatus requires no adjustmentwhen the operating speed of the machine is to be changed.

Referring again to FIG. 2, it will be noted that the waveforms 38 and 46intersect or coincide at a point in time slightly after the time atwhich a missing pulse is detected. In particular, the time ofcoincidence of the signals occurs a short period of time after the timewhen the leading edge of the next pulse would have been present, thetime period being less than the duration of each pulse 24. This isdesigned into the circuitry of the apparatus to provide a predeterminedmeasure of tolerance for slight variations in the duty cycle of pulses24 and for slight slippages of the chain and sprocket drive between themachine or press drive shaft and the shaft of the switching apparatus.Furthermore, there may be some lower limit on the speed of rotation ofshaft 13, for example 10 rpm, below which the circuit of the presentinvention functions as if shaft 13 were completely stopped. Theparticular speed limit will of course depend on the nature of thecircuit. Thus, in defining the present invention the term stopped isintended to include both a complete stopping of the machine element andan effective stopping thereof where the speed falls below the lowerlimit established by the circuitry.

FIG. 3 illustrates one form of electrical circuit which can be utilizedin the system of FIG. 1. In the circuit of FIG. 3 a line 56 is connectedthrough a resistor 57 to a source of positive bias voltage (not shown),and a line I 58 is connected at zero or reference voltage potential.

The pulses 24 generated in response to rotation of cam 14 by shaft 13are applied through a filter comprising a resistor 60 and capacitor 61to the input of an ac. coupled switch circuit 63. Circuit 63 includes atransistor switch 64, the emitter terminal of which is connected to line58 and the base terminal of which is coupled through a capacitor 65 tothe junction of capacitor 61 and resistor 60. The parallel combinationof a diode and resistor are connected between the base of transistor 64and line 58. The collector of transistor 64 is connected throughresistors 66, 67 to line 56. A capacitor 68 couples the output ofcircuit 63 from the junction of resistors 66, 67 to the input of a rampgenerator and clamp circuit 69. The voltage waveform appearing at thebase of transistor 64 has a small negative level when no pulse 24 ispresent, increases instantaneously to a small positive level sufficientto turn on transistor 64 when a pulse 24 appears, decreasesexponentially to zero before pulse 24 disappears, and then changesinstantaneously to the negative level when pulse 24 disappears.Initially the voltage at the junction of resistors 66, 67 is near thevoltage on line 56, but when transistor 64 is'turned on the voltage atthis junction changes instantaneously to zero. It remains at this leveluntil transistor 64 is turned off whereupon the voltage returns to theinitial level. The ramp generator and clamp circuit 69 of FIG. 3includes a transistor 70, the emitter of which is connected to line 58and the base of which is connected to the cathode of a diode 71, theanode of which is connected to capacitor'68. The base terminal oftransistor 70 also is connected through a resistor to line 58. Thecollector terminal of transistor 70 is connected through resistors 72,73 to bias line 56, and the junction of resistors 72, 73 is connectedthrough a capacitor to reference potential line 58, The waveformappearing at the junction of resistors 72, 73 grows from a zero orreference voltage level exponentially to a level approaching the valueof the bias voltage on line 56 whereupon at the beginning of a pulse 24the waveform decreases instantaneously to the zero or reference leveland remains at that level for the duration of the pulse 24. When thepulse disappears, the waveform again grows exponentially to its peakvalue.

The signal at the junction of resistors 72, 73 is applied to a sampleand hold circuit 74 including a transistor 75, the emitter of which isconnected to the anode of a diode 76, the cathode of which is connectedthrough a resistor to a point common to the anode of diode 71 in circuit69 and capacitor 68. The base terminal of transistor 75 is connected toline 58, and the collectorterminal thereof is connected through aresistor to the control or gate terminal of a field effect transistor 78the source terminal of which is connected by lines 79, 80 to thejunction of resistors 72, 73. The waveform apearing at the collector oftransistor 75 is a square wave which duplicates the waveform defined bypulses 24. The waveform appearing at the drain terminal of transistor 78is a sawtooth waveform having a maximum amplitude corresponding in timeto the leading edge of a pulse 24 and which decreases linearly in aregular manner until the next pulse 24 appears whereupon itinstantaneously rises to the maximum value.

The junction of resistors 72, 73 is connected by line 80 to one input ofa comparator circuit 82, the other input of which is connected by a line84 to the drain terminal of transistor 78. Comparator circuit 82provides an output signal on a line 86 only when there exists apredetermined relationship between the two signals applied to the inputsof circuit 82. Line.86 can be connected to the base terminal of atransistor, the emitter terminal of which is connected to line 58 andthe collector terminal of which is connected to the control or gateterminal of a semiconductor controlled rectifier such as a Triac. Thecontrolled rectifier, in turn, can be connected between line 58 and acircuit for energizing a relay for controlling theoperation of themachine such as causing emergency stopping of a power press.

FIGS. 4 and illustrate an alternative form of electrical circuit whichcan be employed in the system of the present invention. Portions of thesystem of FIG. 4 are similar to the system of FIG. 1 and, in particular,a power supply 22 is connected through cam-operated switch 18 to theinput of a pulse shaping circuit 28, the output of which is connected tothe input of sample signal generator 34. The output of generator 34 isapplied to the input of reset signal generator 42, the output of whichis coupled through a time-to-voltage converter circuit 100 to one inputof a peak sample and hold circuit 102. The output'of sample signalgenerator 34 also is connected by a line 103 to the other input ofcircuit 102. Two signals present on lines 105, 106 are applied to acomparator circuit 108, the output of which is connected to load controlcircuit 54.

FIG. 5 is a schematic circuit diagram illustrating portions of thesystem of FIG. 4 in further detail. Power supply 22 comprises thecombination of an ac. source 110 including a conventional transformerand a rectifier 112. Positive and negative bias voltages, for exampleabout 14 volts positive and about 14 volts negative are present on lines114 and 115, respectively connected to the output of rectifier 112, andfor convenience in illustration the connections of lines 114 and 115 toappropriate locations in the circuit of FIG. 5 are shown fragmentarily.A line 117 comprises a reference potential line for the circuit of FIG.5 and is connected to a center tap on the secondary winding of thetransformer includes in source 110. Cam operated switch 118 in thepresent example is connected between a resistor 119 and the common orreference end of the primary winding of the transformer of source 110.As a result, pulses similar to those designated 24 in FIG. 2 appear atthe junction 120 between resistor 119 and a resistor 121. The pulses areapplied to the input of pulse shaper 28 which in the presentillustration comprises a Schmitt trigger circuit. The Schmitt triggercircuit includes a pair of AND gates 122 and 124, and the circle presentat the output end of each gate means that the signal produced thereby isrelatively negative when it represents a logical one. In other words,when there are two relatively positive signals present at the input ofthe gate, a relatively negative signal is produced at the output, and atall other times a relatively positive level is present at the output.Pulses similar to those designated 30 in FIG. 2 are present at theoutput 126 of wave shaping circuit 28, and these pulses are coupledthrough a capacitor 128 to the input of sample signal. generator 34. Inresponse to the presence of each pulse 24, there appears at the output130 of sample signal generator 34 a signal which varies in a regular,continuous manner, similar to that of the signal 38 shown in FIG. 2,until the next pulse 24 occurs whereupon the signal is repeated andagain varies in the same manner as before.

The output of sample signal generator 34 is coupled through a capacitor132 to the input of a reset signal generator 42, and the signal producedby generator by generator 42 varies in a regular, continuous mannersimilar to that of the signal 46 shown in FIG. 2 until the next pulse 24occurs whereupon the signal is repeated. The signal provided bygenerator 42 differs from that provided by generator 34 but in a mannerwhereby there exists readily comparable relationship between the twosignals. The output of reset signal generator 42 is connected to theinput of time-to-voltage converter circuit 100 for converting signalvariations functionally related to time to corresponding variations involtage. The output of converter circuit 100 is applied to the input ofpeak sample and hold circuit 102. Output 130 of sample signal generator34 is connected by line 103 to a portion of circuit 102 including acapacitor and network of resistors 141, and 142 and this portion isconnected through a resistor 145 and by a line 147 to one input ofcomparator circuit 50 including an operational amplifier 150. The otherinput of comparator circuit 50 is connected to another portion of sampleand hold circuit 102. Comparator circuit 50 produces an output signal ona line 152 only when there exists a predetermined relationship betweenthe two input signals applied to operational amplifier 150.

The output of comparator circuit 50, present on line 152, is connectedto a load control circuit designated 54 in FIG. 4, which can include acontrolled rectifier such as a Triac, the control or gate terminal ofwhich would be coupled through a suitable driving circuit to line 152,and which controlled rectifier would be connected in series between thecontrol winding of a relay and the power supply 22. The relay, in turn,controls switch contacts in a machine control circuit such as theemergency stop circuit of a power press control.

Iclaim:

1. In combination with a machine having an element which is rotatedduring operation of said machine, means for detecting when said rotatingelement stops comprising:

a. pulse generating means mechanically coupled to said element forproducing electrical pulses at a frequency directly proportional to thespeed of rotation of said element;

b. signal generating means connected to said pulse generating means forproducing, in response to the occurrence of each pulse, first and secondsignals each varying with time in a regular, continuous manner until thenext pulse occurs whereupon said signals are repeated and again vary inthe same manner as before; and

c. comparison means connected to said signal generating means forproviding an indicating signal in response to the occurrence of apredetermined relationship between said first and second signalsindicative that rotation of said element has stopped.

2. The combination according to claim 1, further including meansconnected to said comparison means and to said machine for controllingthe operation of said machine in response to said indicating signal.

3. The combination according to claim 1, wherein said'element is a camrigidly secured to the shaft of rotary switching apparatus coupled tothe drive of said machine for controlling said machine.

4. The combination according to claim 3, wherein said machine is a powerpress and said rotary switching apparatus is connected to electricalcircuits for controlling the ram clutch and brake of said press.

5. The combination according to claim 4, further including meansconnected to said comparison means and to said press control circuitsfor stopping movement of the ram of said press in response to saidindicating signal.

6. The combination according to claim 1, wherein said cam has aplurality of lobes or rise portions at equally spaced locations aroundthe periphery thereof.

7. The combination according to claim 1, wherein said first and secondsignals produced by said signal generating means decrease and increase,respectively, with time in a linear manner toward the same level ofmagnitude and in a manner whereby said signals do not coincide as longas said pulses continue to appear.

8. The combination according to claim 7, wherein said first and secondsignals coincide if the next pulse fails to occur at its regular time,the time of coincidence of said signals occuring a short period of timeafter the time when the leading edge of the next pulse would have beenpresent, said time period being less than the duration of each of saidpulses.

1. In combination with a machine having an element which is rotatedduring operation of said machine, means for detecting when said rotatingelement stops comprising: a. pulse generating means mechanically coupledto said element for producing electrical pulses at a frequency directlyproportional to the speed of rotation of said element; b. signalgenerating means connected to said pulse generating means for producing,in response to the occurrence of each pulse, first and second signalseach varying with time in a regular, continuous manner until the nextpulse occurs whereupon said signals are repeated and again vary in thesame manner as before; and c. comparison means connected to said signalgenerating means for providing an indicating signal in response to theoccurrence of a predetermined relationship between said first and secondsignals indicative that rotation of said element has stopped.
 2. Thecombination according to claim 1, further including means connected tosaid comparison means and to said machine for controlling the operationof said machine in response to said indicating signal.
 3. Thecombination according to claim 1, wherein said element is a cam rigidlysecured to the shaft of rotary switching apparatus coupled to the driveof said machine for controlling said machine.
 4. The combinationaccording to claim 3, wherein said machine is a power press and saidrotary switching apparatus is connected to electrical circuits forcontrolling the ram clutch and brake of said press.
 5. The combinationaccording to claim 4, further including means connected to saidcomparison means and to said press control circuits for stoppingmovement of the ram of said press in response to said indicating signal.6. The combination according to claim 1, wherein said cam has aplurality of lobes or rise portions at equally spaced locations aroundthe periphery thereof.
 7. The combination according to claim 1, whereinsaid first and second signals produced by said signal generating meansdecrease and increase, respectively, with time in a linear manner towardthe same level of magnitude and in a manner whereby said signals do notcoincide as long as said pulses continue to appear.
 8. The combinationaccording to claiM 7, wherein said first and second signals coincide ifthe next pulse fails to occur at its regular time, the time ofcoincidence of said signals occuring a short period of time after thetime when the leading edge of the next pulse would have been present,said time period being less than the duration of each of said pulses.